1. Field of the Invention.
This invention relates to localized radiation heating and more particularly to localized heating in microwave appliances.
2. Description of the Prior Art.
In microwave heating, it can be desirable to provide localized surface heating to achieve such effects as browning and crisping. While the typical microwave oven is a suitable energy source for uniform cooking, it is not satisfactory for selective heating effects, such as browning and crisping. In fact, the typical microwave arrangement produces the cooking in which the external surface of the cooked material, particularly if desired to be crispy, tends to be soggy and unappetizing in appearance.
One attempt to provide suitable browning and crisping of microwave cooked foods has been by the selective use of virtually transparent, very thin metallized aluminum deposition on a carrier. Such material can produce heat and provide the desired crisping. The difficulty with this thinness of metal is that it can produce arcing and fuses out prematurely, thereby defeating the microwave operation. Arcing is manifested by visible electric sparks which appear on the metal surface.
A prior art susceptor of the type employing a surface coating of vacuum metallized aluminum is illustrated by the laminate of FIG. 4. In this laminate (24), a 1/2 mil (0.013 mm) layer or film of polyethylene terephthalate is used as a carrier (20). Upon this is deposited a 15-20 angstroms thickness of vacuum-metallized aluminum (21) that provides a surface resistivity varying between 20 and 50 ohms per square. Overlying the aluminum layer is an adhesive (22) such as ethylene vinyl acetate and an overlying cellulosic layer (23). When exposed to microwave radiation this susceptor heats up but soon shuts off like a fuse and therefore cannot be reused. During the heating cycle this susceptor has been known to produce arcing.
Another attempt to provide browning and crisping in a microwave oven has been by the use of metal filled polymeric coatings, especially aluminum flake filled coatings as in prior art such as European patent application No. 87301481.5 publication number 0 242 952, published Oct. 28, 1987. These coatings do provide heating upon microwave radiation exposure but the high degree of loading or coating thickness needed to achieve browning temperatures makes the coating prone to arcing.
In European patent application publication No. 0242952 published Oct. 28, 1987, a composite material for heat absorption of microwave energy is disclosed. The disclosed composite material is composed of a dielectric substrate such as polyethylene terephtalate film, coated with an electrically conductive metal or metal alloy in flake form, preferably aluminum flakes, in a thermoplastic dielectric matrix, e.g., a polyester copolymer.
Another attempt to provide the desired heating effect has been by the suggested use of carbon black coatings. These do not produce arcing but are generally found to be unsatisfactory because they produce uncontrolled, extreme run-away heating effect.
In U.S. Pat. No. 4,518,651 a susceptor material composed of carbon filled coating is disclosed. The susceptor material is composed essentially of carbon dispersed polymeric matrix. This reference does not employ metallic components in the susceptor coating. The disadvantage of the carbon based coating disclosed is that it tends to heat too rapidly and can cause ignition of the paperboard substrate cited, known in the art as thermal runaway. Thus, susceptor products of the type disclosed, while effective in terms of their heating properties, can cause hazards especially if the microwave oven is not very carefully monitored.
In U.S. Pat. No. 4,190,757, a susceptor composed of metallic oxide such as iron oxide or zinc oxide is disclosed. This reference also discloses that dielectric materials such as asbestos some fire brick, carbon and graphite can be employed in the susceptor energy absorbing layer. (Col. 7, lines 27 to 51). The reference does not disclose combinations of components other than combinations employing iron oxides for the energy absorbing layers or any advantages to be gained from combinations not utilizing the iron oxides. The reference is thus directed towards use of an iron oxide based coating for the energy absorbing layer. The iron oxide coating thickness is high, namely of the order of 1/16 to 1/8 inch (1.6 to 3.2 mm) which makes it impractical for use in conventional food packaging. Food packaging having such high coating thickness is costly to manufacture and would thus add considerably to the overall cost of the food product.
In U.K. patent publication GB No. 2186478A published Aug. 19, 1987, microwave energy absorbing decals for use on ceramic or glass-ceramic cookware untensils is disclosed. The decals are fused to the ceramic cookware. The decals have an energy absorbing layer which contain at least one metallic oxide and at least one metal in the unoxidized or reduced state. In preferred embodiments, the susceptor material can include iron oxides, nickel oxides and intermetallic oxides of iron and nickel such as nickel-iron ferrite and also can include nickel in the reduced state. The metallic oxides are selected from oxides of iron, nickel and zinc. The metal in the reduced state is selected from iron, nickel or zinc or their alloys. The decals are specifically intended for use on ceramic or glass-ceramic cookware and is not intended for use on paper or plastic packages due to the runaway heating produced.
This reference is not concerned with or directed towards use of an energy absorbing material for food packages, but rather the energy absorbing decals disclosed therein are designed for direct application to ceramic cookware.
Accordingly, it is an object of the invention to facilitate the selective heating of objects, particularly food. A related object is to improve the taste and texture of microwave heated foods. Another object is to maintain the wholesomeness and nutritional value of food.
A further object of the invention is to overcome the disadvantages experienced in the use of vapor deposited metallic coatings in attempting to supply a supplemental heating effect in microwave cooking.
Another object of the invention is to surmount the disadvantages experienced in the use of metal filled polymeric coatings in the attempt to furnish auxiliary heating in microwave cooking.
Still another object of the invention is to overcome the disadvantages that have been experienced in obtaining localized heating effects. A related object is to overcome the difficulties particularly unmanageable runaway heating that have prevented carbon black coatings from being used for localized heating.